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I was meant to find sum to n terms of this:

1, (4-a)x, (7-a^2)x^2, (10-a^3)x^3, (13-a^4)x^4........

Would the correct way to go about it be expand it:

1, 4x-ax. 7x^2-a^2x^2.....

and then it would be:

Sn = (power series of 1, 4x, 7x^2.....) - ax(1-ax^(n-1)/(1-ax)